Fluorinated homopolymers exhibit unique properties that are not observed with other organic polymers. Fluorinated homopolymers possess high thermal stability, chemical inertness, low flammability, low coefficient of friction, low surface energy, low dielectric constant, weather resistance, and gas barrier properties. These properties enable their use in aerospace, automotive, construction, medical, pharmaceutical, and semiconductor industries, for example as packaging films. However, fluorinated homopolymers have some drawbacks. They are often highly crystalline with high melting points, poorly soluble in common organic solvents, and not easily cured or cross-linked. Further, fluorinated homopolymers do not adhere strongly to most surfaces and are known for their non-stick characteristics. As such, processing fluorinated homopolymers is difficult because they lack solubility in common organic solvents that are typically used to apply polymers to various substrates, their high melting points result in application temperatures that may harm the substrate to which they are applied, and they lack adhesion to common substrates.
In contrast, fluorinated copolymers, which are derived from fluoroolefins and functional monomers, are known in the art and have been found to combine the above-noted beneficial properties of fluorinated homopolymers, but without the drawbacks due to the properties of the functional monomers. Such materials can be used as barrier materials, surfactants, polymer electrolytes for Lithium ion batteries, paints and coatings, cores and claddings for optical fibers, chemical sensors, solid polymeric superacid catalysts and ion-exchange and gas separation membranes. There are three principal methods to make fluorinated copolymers with functional monomer groups: (a) polymerization of functionalized fluoromonomers, (b) copolymerization of fluoroolefins with functionalized monomers, and (c) modification of common fluoropolymers by graft and block copolymerization with functional monomers. Chemical modification of common fluoropolymers is also possible in some instances. Because of their limited solubility and chemical inertness, however, there is a very limited range of reagents that can affect fluoropolymers, and only a very limited number of functional groups that can be incorporated with fluoropolymers in this manner. Further, homopolymerization of functionalized fluoromonomers is very limited due to the difficulty in synthesis and high cost of functional fluoromonomers.
The monomers that comprise copolymers can be distributed in one of five general ways, randomly, as regularly alternating series, graft, gradient or as blocks of identical monomers. The type of distribution can be controlled with the choice of polymerization catalyst, the reactivity ratios of both comonomers, chain transfer agent, and reaction conditions.
Block copolymers comprise different polymer blocks linked together by a covalent bond. Diblock copolymers have two polymer blocks connected together whereas triblock copolymers comprise a central polymer block of one type, the ends of which are attached to polymer chains of another type. The components of the block copolymer may be either compatible or incompatible, depending on their chemical structure. The importance of block copolymers comes from their unique chemical structure that brings new physical and thermodynamical properties related to their solid-state and solution morphologies. Several block copolymers have produced a wide range of materials with tailorable properties depending on the nature and length of blocks.
Block copolymers containing vinylidene fluoride blocks have been reported in the literature. In contrast, block copolymers having CTFE blocks are difficult to synthesize and characterize and require tedious isolation procedures due to the insolubility of CTFE blocks in the polymer. In light of these difficulties, a controlled radical polymerization process has been developed to prepare a macro-initiator that is subsequently reacted with CTFE to form di- and triblock copolymers.
Thus, many fluorinated copolymers have been produced by radical copolymerization. Polychlorotrifluoroethylene (PCTFE), for example, is prepared by free-radical polymerization of chlorotrifluoroethylene (CTFE) in an aqueous emulsion or suspension using organic or water-soluble initiators. The emulsion polymerization method requires a surfactant to form a stable emulsion between the monomer and polymer. Most surfactants are fluorinated compounds with a polar head group, and removal of the surfactant is an important part of the synthesis process. Complete removal of the surfactant is very difficult depending on the extent of adsorption to the polymer particles. Further, recent studies indicate that these surfactants are bioaccumulable, toxic, and do not readily biodegrade in the environment.
PCTFE is also often produced using suspension polymerization, which employs redox initiators such as metal persulfates and bisulfites with iron or copper salts as catalysts. The PCTFE polymer produced by this method exhibits poor thermal stability (tendency to change chemically) during film extrusion at temperatures in the range of about 275-325° C. The poor stability is attributed to ionic end groups, which can undergo hydrolysis during synthesis to form unsaturated olefins and carboxylic acid. That hydrolysis is followed by a decarboxylation that further generates some unzipping (or depolyerization) which decomposes PCTFE. Thus, thermally-pressed PCTFE samples prepared by the suspension polymerization method often undesirably show bubbles and discoloration, which is thought to be due to oligomers formed at the end of the polymerization when the reaction is pushed to high conversion.
Regardless of the method employed, PCTFE also has a strong tendency to crystallize, and thus molecular weights must be kept sufficiently high to maintain the desired degree of crystallinity for optimum physical and mechanical properties. In order to develop the physical properties necessary for most end use applications, the polymerization must be controlled. This can be achieved by proper processing conditions with chain transfer agents (CTA). Perfluoroalkyl iodides and α,ω-diiodoperfluoroalkanes have been employed as chain transfer agents in iodine transfer polymerizations. These iodides are expensive, toxic, and often insoluble in water.
In order to overcome the deficiencies exhibited in the prior art, it would be desirable to provide improved methods for the production of fluorinated copolymers. Further, there remains a need in the art for cost effective packaging films having the moisture barrier properties that meet present and future performance demands. Still further, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.